Process for electrostatic precipitation



c. c. sHALE PROCESS FOR ELECTROSTATIC PRECIPITATION Dec. 9, 1969 Filed Oct. 5, 1967' 2 Sheets-Sheet 1 G uvvnvron GORRELL 6. SHALE ATTORNEY Dec. 9, 1969 QSHA'LE I 3,482,374

PROCESS FOR ELECTROSTATIC PRECIPITATION Filed Oct. 5, 1967 2 Sheets-Sheet 2 O I I I I 20 4O 6O 80 I00 I20 PIPE DIAMETER/WIRE DIAMETER, D/CI PRESSURE I ATM.

0 I I 1 I l 2O 4O 6O 8O I00 I20 PIPE DIAMETER/ WIRE DIAMETER, D/d

PRESSURE 3ATM.

INVENTOI? COIIWEZL 6. .S'HALE ATTORNEY United States Patent PROCESS FOR ELECTROSTATIC PRECIPITATION Correll C. Shale, Morgautown, W. Va., assignor to the United States of America as represented by the Secretary of the Interior Filed Oct. 3, 1967, Ser. No. 672,647 Int. Cl. B03c 3/40, 3/00 US. Cl. 552 4 Claims ABSTRACT OF THE DISCLOSURE Electrostatic precipitation of particulate matter from gas streams above about 1200 F. and at low to intermediate pressures is accomplished by providing a collecting electrode to emitting electrode effective diameter ratio of less than about 50.

This invention resulted from work done by the Bureau of Mines of the United States Department of the Interior, and the domestic title to the invention is in the Government.

Background of the invention Electrostatic precipitators are widely used for the removal of solid and liquid particulate material from flowing gas streams. Single stage, or Cottrell-type precipitators which utilize a combined charging and collecting zone, have proved to be superior for cleaning industrial furnace and process gas.

The simplest form of the Cottrell-type precipitator is the pipe-type which consists of a fine wire emitting electrode surrounded by and coaxial with a cylindrical c01- lecting electrode. In operation, the wire is given a high voltage negative charge while the surrounding collecting electrode is held at ground potential. The impressed voltage is sufficiently great to cause a corona which ionizes the gas in the immediate vicinity of the wire. Negative ions then flow toward the collecting (positive) electrode, impact upon particles suspended in the gas, charge the particles and cause them to migrate and be removed from the gas by adherence to the grounded collecting electrode.

The operating voltage range of an electrostatic precipitator is the difference between the voltage at which ions are first created, called the corona starting voltage, and the voltage which produces a continuous are between the two electrodes which is called the sparkover voltage. Corona starting and sparkover voltages are a function of gas composition, gas temperature and pressure, the distance between the emitting and collecting electrodes and the diameter ratio of the collecting electrode to the emitting electrode. In a pipe-type precipitator, this ratio is expressed as D/d where D is the diameter of the pipe (ground or collecting electrode) and d is the diameter of the wire. Precipitators utilizing large values of D/d, that is 100 or greater, provide excellent electrical characteristics but the emitting electrode wire is so fine that the unit is rather fragile. Industrial design is a compromise between the opposing considerations of desirable electrical characteristics and mechanical strength and generally utilizes a D/d ratio of about 70.

Operating conditions of industrial electrostatic precipitators include temperatures as high as about 1200 F., pressures up to about 150 p.s.i. and gas flows as high as 3,000,000 c.f.m. High temperature operation compounds operating difficulties of electrostatic precipitators. Plate and duct type collecting electrodes are subject to warping at high temperatures and so are less suitable than is the pipe-type precipitator. As temperature increases, corona current rises and sparkover voltage drops rapidly thus narrowing the operating voltage range.

Sparkover voltage also shows a relatively linear dependence upon gas density, decreasing as gas density decreases. Since gas density is temperature dependent, providing that pressure remains constant, this relationship also tends to decrease the operating voltage range of an electrostatic precipitator when used at high temperatures.

The collection efiiciency of an electrostatic precipitator is governed primarily by the precipitating voltage; efiiciency increasing roughly as the square of the precipi tating voltage. In a single stage precipitator, the precipitating voltage is the same as the charging voltage across the electrodes. For example, if the precipitator efiiciency were at a relative precipitator voltage of 1.0, then the efiiciency would be on the order of 60% at a relative voltage of 0.9 and would be over 99% at a relative precipitator voltage of 1.2. For this reason, electrostatic precipitators are customarily operated at voltages just below the sparkover point. An excellent theoretical treatment of electrostatic precipitation can be found in H. J. Whites book Industrial Electrostatic Precipitation, Addison-Wesley Publishing Co., 1963.

As has been discussed previously, high temperatures decrease the sparkover voltage thus drastically reducing the attainable efiiciency of an electrostatic precipitator. The decrease in gas density with increasing temperature, provided the pressure remains constant, further decreases sparkover voltage and thus compounds the difiiculties of high temperature operation. For these reasons, electrostatic precipitation becomes less and less attractive and useful as the temperature of the gas being cleaned rises.

The present invention provides a process and apparatus for the electrostatic separation of particulate matter from high temperature and relatively low density gas streams. Thus the primary object of this invention is to provide means to allow use of an electrostatic precipitator at high temperatures and low to intermediate gas densities.

Another object of this invention is to allow use of electrostatic precipitators at conditions which heretofore have been impossible.

A further object of this invention is to conserve sensible heat in hot, dust-laden process gases for further utilization by removing the entrained solids at high temperat-ures.

Description of the invention The invention will be more clearly understood from the following description of a preferred embodiment wherein reference is made to the accompanying d-rawmgs:

FIG. 1 is a partial sectional view of a pipe-type electrostatic precipitator adapted for high temperature use.

FIG. 2 is a graphical presentation of the relationship of operating voltage range to the D/d ratio at selected temperatures with a gas pressure of 1 atmosphere.

FIG. 3 illustrates the effect of increased pressure on the operating voltage range.

Referring now to FIG. 1, there is shown a precipitator construction found to be satisfactory for high temperature use. Outer collecting electrode 1 preferably com prises an elongated hollow cylinder provided with a gas entry port 2 and a clean gas exit port 3. While a cylindrical or pipe-type configuration is preferred, other symmetrical geometric shapes may be employed. For example, the collecting electrode may be of hexagonal cross-section, but such shapes are less resistant to warping and distortion at high temperatures than are cylinders.

This electrode is held at ground potential by any suitable grounding means 4. The cylindrical or pipe-like electrode 1 is flanged at top and bottom and connected to a cathode entry chamber 5 and a dust particle collection chamber 6. Cathode entry chamber 5 may be cooled by means of a tubular coil 7 through which water or any other appropriate heat exchange medium flows, entering at 8 and leaving the coil at 9. Similarly, particle collection chamber 6 may be cooled by coil 10 by means of flowing water which enters at 11 and leaves at 12. While it is not necessary to the operation of the precipitator to provide cooling for the cathode entry and particle collection chambers, such a provision allows greater flexibility in construction and operation of the device.

Disposed within and coaxial to the outer positive collecting electrode is cathode 13. The cathode is preferably a circular wire of such size as to provide a diameter ratio (D/d) of the collecting electrode 1 to the emitting electrode 13 of less than about 50 and most preferably of less than about 30. The diameter d of the emitting electrode, or cathode, is here defined to be its elfective emitting diameter. Thus, in a cathode consisting of a rectangular, triangular or other geometric shape, the effective diameter is defined by the radii of curvature making up the corners of those geometric shapes.

Cathode 13 is supported and centered within collecting electrode 1 by means of insulator 14. Insulator 14 has two functions in that it acts as a gas seal as well as electrically insulating the cathode from the body of the precipitator. Ceramics such as porcelain have been found to be satisfactory materials for insulator construction.

The cathode terminates below the gas entry port and attaches to a second insulator 15. Weight 16 is suspended from the second insulator so as to maintain the cathode under tension. Vertical alignment of the cathode is maintained by spider 17.

An adjustable, high voltage, negative potential is produced by power supply 18 and is transmitted to the emitting electrode 13 by means of cable 19. Current to operate the high voltage D-C power supply is provided by A-C input 20.

Referring now to FIGS. 2 and 3, these graphs illustrate the reversal of normal electrode size to operating voltage range relationships which occurs at temperatures above 1200 F. In each of the curve sets presented, the capital letter identifies the sparkover voltage while the corresponding primed capital letter identifies the corona onset voltage. For example, in FIG. 2, the line A represents sparkover voltage at various D/ d ratios at 600 F. while the line A represents the corresponding corona onset voltage. The difference between lines A and A represents the operating voltage range available at any particular D/d ratio. The data presented was obtained using a 2-in. diameter collecting electrode. Use of larger diameter collecting electrodes gives similar corona onset and sparkover voltage relationships. However, with larger collecting electrodes the curves are shifted upward to higher voltage levels.

It has long been recognized in the art that electrical characteristics of a precipitator improve and the operating voltage range increases as the cathode wire electrode is made smaller relative to the collecting electrode. Curves A-A' and BB illustrate this normal relationship. At a D/d ratio of 19, sparkover occurs before corona onset while high D/d ratiosprovide a relatively large operating voltage range.

Examination of curves C-C' shows that at 1500 F., the normal relationship is completely reversed. At this temperature, only relatively large cathode wire electrodes provide an adequate operating voltage range. As the cathode wire size decreases, thus increasing the D/ d ratio, the operating voltage range steadily shrinks until, at a D/d ratio of 121, sparkover occurs before corona onset. This reversal of normal electrode relationships occurs at temperatures above 1200 F. and is completely developed at 1350 F. As shown by the curves C-C', D/d ratios smaller than about 50 are particularly advantageous at temperatures of about 1500 F. and atmospheric pressure.

FIG. 2 further illustrates that at 600 F. it is advantageous to operate at the largest possible D/d ratios since such ratios provide the highest sparkover voltage and the greatest operating range. This is in agreement with the accepted teachings of the prior art. At 1200" F. as shown by curves B B', sparkover voltage increases with decreasing D/d ratios; a behavior which is opposite to that displayed at lower temperatures. This trend becomes much more pronounced with increasing temperature as shown by curves C-C'.

The gain in efiiciency realizable by operating an electrostatic precipitator according to the teachings of this invention is readily evident. For example, at 1500 F., and 1 atm. pressure, the maximum operating voltage (sparkover) using a D/d ratio of 20 is about 2 /2 times that attainable by using a conventional D/d ratio of about 70. This increase in maximum operating voltage increases precipitator efliciency by a factor of 6 to 7 since efiiciency increases roughly as the square of the precipitating voltage.

A comparison of FIGS. 2 and 3 illustrates the effect of pressure on the process. Increasing the operating pres sure increases the effective density of the gas being treated and this leads to a broadening of the operating voltage range. This relationship holds true for the entire temperature and D/ d ranges investigated. As shown by curves E- E, a D/d ratio of 19 provides a small operating voltage range at 1200 F. as does a D/d ratio of 121 at 1500 F. Thus it is evident that the operating advantages of this invention are maximized at low to intermediate gas pressures and densities.

No explanation for the unexpected reversal of electrode size to operating voltage range relationships found to occur at temperatures above 1200 F. is available at this time. One suggested explanation is that a diiferent ion-forming mechanism becomes dominant at high temperatures but the inventor does not wish to be bound by any particular theory of application. I

Example A particle-containing gas at a temperature of about 1500 F. and a pressure of 1 to 3 atmospheres enters the precipitator at port 2. The gas is subjected to an electrical field maintained just below the sparkover voltage between collecting electrode 1 and emitting electrode 13. The ratio of electrode diameters, D/ d is fixed at less than about 50 and preferably about 20-25. The particles contained in the gas are subjected to intense bombardment by negative ions and become highly charged. The charged particles are driven to the wall of the positive electrode by the intense electric field where they remain held. Cleaned gas leaves the precipitator by way of port 3. Cleaning of the collecting electrode is accomplished in a conventional manner; either by intermittent or continuous rapping of the tube or by sequential operation coupled with high temperature steam or air flushing.

While only one precipitator unit has been illustrated, it is readily apparent that any number of units can be operated in parallel as is conventionally done in the art so as to handle large gas streams.

While the illustration of the invention has been restricted to its use in electrostatic precipitation, it is obvious that the process could also be used to control process current and/or voltage from a high temperature ion source. It will be understood that a number of such variations and adaptations of the disclosed invention are possible without departing from its spirit or scope.

What is claimed is:

1. A method for increasing the operating voltage range of an electrostatic precipitator operated above about 1200" F. comprising:

(a) providing an electrostatic precipitator having an emitting and a collecting electrode;

(b) passing a hot gas stream containing particulate matter and having a temperature above about 1200 F. through said electrostatic precipitator and between said emitting and said collecting electrode;

(0) establishing a voltage gradient between said emitting and said collecting electrode;

((1) extending the operating voltage range of said electrostatic precipitator beyond the operating voltage range obtainable with a collecting electrode to emitting electrode efiective diameter ratio of more than about 50 by providing a collecting electrode to emitting electrode eifective diameter ratio of less than about 50;

(e) maintaining said emitting electrode at substantially the temperature of said hot gas stream; and

(f) precipitating by action of said voltage gradient said particulate matter on said collecting electrode.

2. The process of claim 1 wherein said precipitator comprises an outer cylindrical collecting electrode and a coaxial wire emitting electrode, and said hot gas stream is at a temperature above about 1350 F.

3. The process of claim 1 wherein said hot gas stream is at a temperature above about 1350 F. and a pressure below about 3 atmospheres absolute.

4. The process of claim 1 wherein said hot gas stream is at a temperature above about 1350 F. and said electrode efiective diameter ratio is about 20.

References Cited UNITED STATES PATENTS 6 1,601,771 10/1926 Rowland -120 1,604,424 10/ 1926 Schmidt 55-9 X 2,903,089 9/1959 Latham 55-130 X 3,054,243 9/1962 Bowie 55-11 3,124,437 3/1964 Lagarias 55-13 3,125,426 3/1964 Herber et al. 55-130 3,157,479 11/1964 Boles 55-146 3,395,193 7/1968 Bruce et al 55-134 X OTHER REFERENCES HARRY B. THORNTON, Primary Examiner DENNIS E. TALBERT, JR., Assistant Examiner US. Cl. X.R. 

